Method for producing a heat exchange tube

ABSTRACT

A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger may include providing an austenitic heat exchange tube and the turbulence insert. The method may also include inserting the turbulence insert into the austenitic heat exchange tube, brazing the turbulence insert and the austenitic heat exchange tube via induction brazing, and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to International Application PCT/EP2017/068377 filed on Jul. 20, 2017, and to German Application DE 10 2016 215 265.3 filed on Aug. 16, 2016, the contents of each are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a method for producing a heat exchange tube having a turbulence insert for a heat exchanger. The invention furthermore relates to a heat exchanger with at least one such heat exchange tube.

For example, a heat exchange tube having a turbulence insert brazed within it is known from DE 102 33 407 B4. Such a heat exchange tube is also known from DE 10 2006 013 868 A1.

BACKGROUND

In general, heat exchangers and, in particular, exhaust-gas heat exchangers should have a high level of robustness against pressure and temperature changes, a high level of power density, a low level of fouling behaviour, as well as a low weight and a low production costs during production and, thereby, at the same time, they should offer a high level of flexibility with regard to use. For these competing objectives, there are different solutions, which, however, always fulfil one of these tasks better than the other. Thereby, none of the known solutions is deemed the leading-edge for all issues.

In the case of heat exchangers and, in particular, in the case of exhaust-gas heat exchangers, usually constructions with and without gas-side ribs are differentiated. The constructions without gas-side ribs (for example, winglet tube bundles) are usually made of a bundle of empty tubes, into which structures are embossed, which generate the turbulences on the inner side and thereby improve heat transfer and, at the same time, reduce the fouling tendency. These tube bundles are consolidated via so-called tube sheets, wherein the tube sheets together with a housing form a space where coolant flows. Depending on the embodiment, diffusers are attached to the tube sheets themselves directly or indirectly over the housing, which form the gas-side flow space. Usually, such heat exchangers are brazed in their entirety or made within the scope of a laser-welding process.

Heat exchangers with a gas-side rib achieve a performance increase consisting of an increase of a secondary surface of the heat exchanger, wherein, in this case, a heat exchanger block made of ribs (turbulence inserts), tubes and tube sheets are brazed in their entirety. Should a housing be necessary, this is also brazed along with or welded to the tube sheets and between the shells having a two-shell design.

Being unfavourable in the case of the heat exchangers known from prior art, however, in particular, the comparably elaborate production of heat exchangers with a gas-side rib and a strength loss due to a so-called soft annealing, which cannot be underestimated and occurs in the case of long-term heating in a brazing furnace for the production of brazing connections.

SUMMARY

The present invention therefore deals with the problem of indicating a production method, by means of which a high-performance heat exchange tube can be easily produced on a technical manufacturing level without, however, experiencing a strength loss during production, which has been the case up to this point.

According to the invention, this problem is solved by means of the object of the independent claim(s). Favourable embodiments are the object of the dependent claim(s).

The present invention is based on the general idea to use and induction-brazing method when producing a heat exchange tube of a heat exchanger, for example, an exhaust-gas heat exchanger, whereby, in particular, brazing in a brazing oven and, connected with this, a soft annealing of the heat exchange tube as well as the strength loss associated therewith and the remaining components of the heat exchange tube can be avoided. The production method according to the invention is characterized by the following method steps: Initially, an austenitic heat exchange tube and a preferably ferritic turbulence insert are provided. In a second method step, the turbulence insert is pushed into the heat exchange tube and then the turbulence insert is brazed to the heat exchange tube by means of induction brazing and thereby affixed. Before, during and/or after the induction brazing, the heat exchange tube can be pressed against the turbulence insert from the outside, thereby being affixed. The brazing can thereby occur by means of the heat exchange tube with the turbulence insert arranged on the inside being led through an inductor and thereby, the heat exchange tube, the brazing alloy and the turbulence insert are heated to such an extent that the brazing alloy melts. By pressing the heat exchange tube onto the turbulence insert, a reliable connection between these two components can be achieved during cooling and after exiting the inductor. Due to the exclusively local application of heat, the heat exchange tube cools considerably more quickly in comparison to a known brazing process in a brazing oven known up until this point, whereby, in particular, the strength loss occurring due to the long-lasting heating in the brazing oven can be reliably avoided.

Thereby, for the turbulence insert, an austenitic stainless steel can be used, for example, if corrosion requirements require certain alloys, such as 1.4404, for example.

In the case of inductive heating of the heat exchange tube, the electromagnetic alternating field, which is incorporated by means of the inductor, is partially or fully absorbed in the interior space of the tube by the preferably ferritic material according to the invention so that heating the brazing alloy only takes place via contact with the heated heat exchange tube. A tube material with a low level of magnetic permeability and a high level of electrical resistance thereby allows the electromagnetic alternating field to penetrate into the interior space of the heat exchange tube. Thereby, the standard penetration depth S is referred to. In the case of this, the field strength has fallen to approx. 37% of the value outside of the heat exchange tube. In the case of an austenitic stainless steel, which is ferromagnetic and has a relatively poor electrical conductivity, this penetration depth is approx. 1 mm or more in the case of frequencies of 50 to approx. 200 kHz, which are usual for inductive brazing. In the case of a wall thickness of the heat exchange tube of a maximum of 0.5 mm, thereby, a direct heating of the brazing alloy or of the turbulence insert is possible. Due to the design of the turbulence insert according to the invention made of a preferably ferritic material (steel), the power coupling of the alternating field is further shifted into the direction of the turbulence insert, which further reduces the temperature differences between the heat exchange tube and the turbulence insert, and, thereby, the inner stress. Thereby, it must be particularly emphasized that a standard penetration depth δ of eddy currents in an austenitic material at the same frequency is considerably greater than in a ferritic material so that, due to the design of the heat exchange tube made of an austenitic material, a good penetration of the same can be ensured by means of the eddy currents while the turbulence insert made of a preferably ferritic material has a worse or a lower standard penetration depth δ, thereby causing a certain level of shielding. By means of this, in particular, a precise local heating of the brazing alloy can be achieved without the whole heat exchange tube and the turbulence insert arranged inside of it having to be heated over the long term, for example, in a brazing oven. Furthermore, the brazing alloy is heated from two sides due to this. Thereby, by means of the production method according to the invention, a heat exchange tube of a heat exchanger, for example of an exhaust-gas heat exchanger can be produced in a high-quality manner without having to accept the decrease in strength occurring in the process due to soft annealing.

In the case of a favourable further embodiment of the solution according to the invention, the brazing alloy is applied to the inner side of the heat exchange tube and/or onto the turbulence insert as a brazing foil or as a brazing paste, in particular, being applied to contact points of the same with the inner side of the heat exchange tube. Both the application of the brazing alloy as a brazing foil as well as applying as a brazing paste represents a high-quality possibility on the one hand and an inexpensive one on the other. In particular, both of these methods can be integrated into a so-called “inline process”.

In accordance with a favourable further embodiment of the solution according to the invention, a pressing of the heat exchange tube onto the turbulence insert takes place during and/or after induction brazing to at least one roller pair or a rigid pressing device. At least after induction brazing via the at least one roller pair, a pressing force is applied to the induction brazing point, which affixes this until the brazing alloy solidifies. In order to accelerate this process, furthermore, it is conceivable to cool the roller pair, whereby the production process can be accelerated, and a cycle time can be reduced.

In the case of a favourable further embodiment of the solution according to the invention, an austenitic stainless steel is used for the heat exchange tube. Such an austenitic stainless steel thereby offers the great advantage that this is both rust-free as well as chemically resistant against exhaust gas and, in addition, can withstand continuous temperatures of up to 600° without subsequent heat treatment.

In the case of a further embodiment of the method according to the invention, the induction brazing takes place subjected to a protective-gas atmosphere. By means of such a protection-gas atmosphere, in particular, oxygen can be kept away from the brazing points, whereby it can be prevented that the oxygen reacts with the brazing alloy or the materials of the turbulence insert and of the heat exchange tube, and, here, forms pores and therefore weak points for example.

The present invention is furthermore based on the general idea of equipping a heat exchange tube with at least one heat exchange tube produced according to an aforementioned method, which is affixed in an associated tube sheet of the heat exchanger via a laser-weld connection. Thereby, the heat exchange tube produced by means of the method according to the invention can furthermore be used in known production processes for tube bundle coolers, thereby compensating for the disadvantages occurring from prior art when brazing in brazing ovens. In particular, the heat exchanger according to the invention thereby makes the production possible without the disadvantages know from prior art with regard to the strength due to soft annealing in the brazing oven. The laser welding of the individual heat exchange tubes to the tube sheet also causes a heat input that is exclusively limited to a local level, thereby causing low levels of thermal load.

In the case of a favourable further embodiment of the solution according to the invention, the heat exchange tube of the heat exchanger comprises at least one longitudinal bead and/or transverse bead. In particular, a reinforcement of the heat exchange tube can be achieved via such longitudinal or transverse beads, whereby a reinforcement of the heat exchanger block thereby produced can be achieved. Such longitudinal beads or transverse beads can be produced in a comparatively simple way on a technical manufacturing level, even in an inline process.

In the case of a favourable further embodiment of the solution according to the invention, a longitudinal-end region of the heat exchange tube welded to the tube sheet is free of brazing alloy. By means of this, it can be prevented that the brazing alloy mixes during laser welding the longitudinal ends of the heat exchange tubes with the associated through openings of the tube sheet, thereby leading to local cracks.

Other important features and advantages of the invention result from the subclaims, the drawings and the related figure description based on the drawings.

It is to be understood that the features explained in the aforementioned and following cannot only be used in the respectively indicated combination, but also in other combinations or alone, without departing from the scope of the present invention.

Preferred exemplary embodiments of the invention are represented in the drawings and will be described in more detail in the following description, wherein the same reference numbers will refer to the same or similar or functionally identical components.

BRIEF DESCRIPTION OF THE DRAWINGS

Thereby, the figures schematically show

FIG. 1 is a cross-sectional illustration through a heat exchange tube produced according to the invention,

FIG. 2 shows an apparatus for carrying out the production method according to the invention,

FIG. 3 is a cross-sectional illustration through a possible embodiment of a heat exchanger according to the invention.

DETAILED DESCRIPTION

In accordance with FIG. 1, a heat exchange tube according to the invention 1 comprises a turbulence insert 2 arranged on the inside. Such a heat exchange tube 1 according to the invention is used in a heat exchanger 3, for example, an exhaust-gas heat exchanger 4 (compare FIG. 3). According to the invention, now, the heat exchange tube 1 is made of an austenitic material, for example, a stainless steel, whereas the turbulence insert 2 is made of a preferably ferritic material, for example, also a stainless steel. The difference between the two materials can thereby only be in their magnetizability. By designing the turbulence insert 2 out of a preferably ferritic material and designing the heat exchange tube 1 out of an austenitic material, in particular, a completely new production method for the heat exchange tube 1 can be made possible, which, in particular, avoids the disadvantages known from prior art, such as a decrease in strength due to soft annealing in a brazing furnace for example.

The heat exchange tube 1 according to the invention is produced as follows:

Initially, the austenitic heat exchange tube 1 and the preferably ferritic turbulence insert 2 are provided, wherein, then, the turbulence insert 2 is pushed into the heat exchange tube 1. Now, a brazing of the turbulence insert 2 to the heat exchange tube 1 takes place by means of induction brazing, for which, in particular, an inductor 5 (compare FIG. 2), for example, an inductor coil 6, is used. Thereby, in FIG. 2, the induction coil 6 is arranged in such a way that the heat exchange tube is moved through it. Thereby, inductors 5 are also conceivable that are arranged above and below and designed as flat coils. Before, during and/or after the induction brazing, a pressing of the heat exchange tube 1 against the inner turbulence insert 2, whereby the brazing connection to be produced can be affixed in a reliable manner. The pressing of the heat exchange tube 1 against the turbulence insert 2 can thereby take place by means of a roller pair 7 (compare FIG. 2) for example. Up until this point, the turbulence inserts 2 are also affixed within the heat exchange tube 1 by means of brazing, wherein, however, the brazing took place in a brazing oven, in which both the heat exchange tube 1 as well as the turbulence insert 2 were heated for hours, thereby experiencing a decrease in strength which was not to be underestimated due to soft annealing. This can now be reliably avoided by means of induction brazing according to the invention since a heat input required for brazing only takes place locally over a short period without the long-term heating of the turbulence insert 2 or of the heat exchange tube 1. A soft annealing does not take place here at all. A considerably quicker cooling of the heat exchange tube 1 after brazing has been completed, which can naturally still be supported in an optimal manner that a roller pair 7 are cooled, for example, being arranged after the inductor 5. For this purpose, a corresponding cooling device is arranged in the roller pair 7 and connected to this.

In the case of inductive heating of the heat exchange tube 1, the electromagnetic alternating field, which is incorporated by means of the inductor, is partially or fully shielded in the interior space of the tube by the preferably ferritic material according to the invention so that heating the brazing alloy only takes place via contact with the heated heat exchange tube 1. In the case of an austenitic stainless steel, which is ferromagnetic and has a relatively poor electrical conductivity, this penetration depth is approx. 1 mm or more in the case of frequencies of 50 to approx. 200 kHz, which are usual for inductive brazing. In the case of a wall thickness of the heat exchange tube 1 of a maximum of 0.5 mm, thereby, a direct heating of the brazing alloy or of the turbulence insert 2 is possible. Due to the design of the turbulence insert 2 according to the invention made of a preferably ferritic material (steel), the power coupling of the alternating field is further shifted into the direction of the turbulence insert 2, which further reduces the temperature differences between the heat exchange tube 1 and the turbulence insert 2, and, thereby, the inner stress levels.

The heat exchange tube 1 according to the invention is used, for example, in an exhaust-gas heat exchanger 4, wherein the individual heat exchange tubes 1 are affixed on their respective longitudinal ends 8 in an associated tube sheet 9 or in passages arranged in this respectively. A fluid-tight fixation of the heat exchange tubes 1 in the associated passages of the tube sheet 9 takes place by means of laser welding, for the purpose of which a laser-welding device 10 is used. By means of this, the production of a heat exchanger block 11 with heat exchange tubes 1 and a longitudinal-end side of the same arranged tube sheets 9 can take place in a conventional way, whereby a comparably robust heat exchanger block 11 can be created without having to fear a decrease in strength, which has taken place up until this point in the brazing furnace.

In order to avoid a negative influence of a brazing alloy 12, which is applied onto an inner side of the heat exchange tube 1 and/or onto the turbulence insert 2, for example, as a brazing foil 13 or as a brazing paste 14, in view of the laser-weld connection 17, preferably, it is provided that a longitudinal-end region 8 of the heat exchange tube 1 welded to the tube sheet 9 is free of brazing alloy. By means of this, in particular, an entry of the nickel-container brazing material to the welding region and, at the same time, also the negative influence associated therewith can be avoided.

Brazing with the induction-brazing method and/or the laser welding used according to the invention can thereby take place subject to a protection-gas atmosphere, which, in particular, prevents an undesired entry of oxygen to the brazing connection or to the laser-weld connection 17.

With the heat exchange tube 1 produced by means of induction brazing and the tube sheets 9 connected to it by means of laser welding, in particular, the soft annealing of the steel in the brazing oven causing the decrease in strength up until this point can be avoided, whereby the entire heat exchanger block 11 and, in particular, also the tube sheets 9 have a higher level of strength over the long term. Due to the laser-weld connection 17 of the heat exchange tubes 1 into the passages of the tube sheets 9, an exclusively local heat input can also be achieved, which, in turn, does not result in soft annealing of the tube sheet 9 or the heat exchange tubes 1 and, by means of this, also does not cause the decrease in strength caused due to this up until this point in the brazing oven. By means of the induction brazing method according to the invention and the laser welding, which is also according to the invention, the extensive thermal load can be considerably reduced when joining the turbulence insert 2 in the heat exchange tube 1 and when joining the heat exchange tube 1 to the related tube sheet 9, whereby the steel microstructure and thereby, also the strength can be maintained. In order to further increase the stiffness of the heat exchange tubes 1, these can also have longitudinal beads 15 (compare FIG. 1) or transverse beads 16.

Thereby, the method according to the invention can particularly be used as a continuous production process, whereby very low lead times and costs for the production of the heat exchange tube 1 with a turbulence insert 2 inductively brazed into it can be achieved in comparison to furnace brazing. Furthermore, a tried and tested tube bundling technology with heat exchange tubes 1 laser-welded into the tube sheets 9 and their known advantages can be maintained.

Thereby, it is conceivable that the provided heat exchange tube 1 is open on its longitudinal side so that the turbulence insert 2 together with a brazing foil 13 can be better inserted/pressed. After inserting the turbulence insert 2, the heat exchange tube 1 is sealed by means of a laser-welding process and then brazed by means of induction brazing. Thereby, the brazing foil 13 can be attached on both sides of the turbulence insert 2, for example, above and below, which makes the production process considerably easier since the problem up until this point has been pressing the brazing foil 13 together with the turbulence insert 2 in a clean manner. This is done with a brazing foil 13, which is folded on the front around the turbulence insert 2, meaning a long brazing foil 13, which is longitudinally laid around the turbulence insert 2 a single time. That has the advantage that the brazing foil 13 initially completely seals the heat exchange tube 1 on one side a single time and has to be melted completely during the brazing process. Thereby, unnecessary brazing alloy 12 is also located in the heat exchange tube 1. 

1. A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger, comprising: providing an austenitic heat exchange tube and the turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube via induction brazing; and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.
 2. The method according to claim 1, further comprising applying a brazing alloy as at least one of a brazing foil and a brazing paste onto at least one of an inner side of the austenitic heat exchange tube and the turbulence insert.
 3. The method according to claim 1, wherein the pressing of the austenitic heat exchange tube onto the turbulence insert is via at least one roller pair.
 4. The method according to claim 1, wherein at least one of: the austenitic heat exchange tube is composed of an austenitic stainless steel; and the turbulence insert is composed of at least one of a ferritic stainless steel and an austenitic stainless steel.
 5. The method according to claim 1, wherein the brazing takes place subject to a protection-gas atmosphere.
 6. A heat exchanger comprising at least one heat exchange tube affixed in an associated tube sheet via a laser-weld connection, the at least one heat exchange tube being produced by: providing an austenitic heat exchange tube and a turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube via induction brazing; and pressing the austenitic heat exchange tube onto the turbulence insert at least one of during the brazing and after the brazing.
 7. The heat exchanger according to claim 6, wherein a wall thickness of the at least one heat exchange tube is smaller than 0.5 mm.
 8. The heat exchanger according to claim 6, wherein the at least one heat exchange tube is a shaped and laser-welded steel strip.
 9. The heat exchanger according to claim 6, wherein the at least one heat exchange tube includes at least one of at least one longitudinal bead and at least one transverse bead.
 10. The heat exchanger according to claim 6, wherein at least one of: a longitudinal-end region of the at least one heat exchange tube laser-welded to the associated tube sheet is free of a brazing alloy; and the heat exchanger is designed as an exhaust-gas heat exchanger.
 11. The heat exchanger according to claim 6, wherein a longitudinal-end region of the at least one heat exchange tube laser-welded to the associated tube sheet is free of a brazing alloy.
 12. The heat exchanger according to claim 7, wherein the at least one heat exchange tube includes at least one of at least one longitudinal bead and at least one transverse bead.
 13. The method according to claim 1, wherein the at least one of during the brazing and after the brazing includes both during the brazing and after the brazing.
 14. The method according to claim 1, wherein the austenitic heat exchange tube is composed of an austenitic stainless steel.
 15. The method according to claim 1, wherein the turbulence insert is composed of a ferritic stainless steel.
 16. The method according to claim 1, wherein the turbulence insert is composed of an austenitic stainless steel.
 17. A method for producing a heat exchange tube having an inner turbulence insert for a heat exchanger, comprising: providing an austenitic heat exchange tube and the turbulence insert; inserting the turbulence insert into the austenitic heat exchange tube; brazing the turbulence insert and the austenitic heat exchange tube subject to a protection-gas atmosphere via induction brazing; pressing the austenitic heat exchange tube onto the turbulence insert via at least one roller pair at least one of during the brazing and after the brazing.
 18. The method according to claim 17, further comprising applying a brazing alloy onto at least one of an inner side of the austenitic heat exchange tube and the turbulence insert.
 19. The method according to claim 18, wherein the brazing alloy is a brazing foil.
 20. The method according to claim 18, wherein the brazing alloy is a brazing paste. 